Researchers evaluate affordable bridge repair method
U of M researchers recently wrapped up an evaluation of a cost-effective technique for repairing damaged ends of reinforced concrete bridge girders.
Although these girders can bear heavy loads well for decades, girder ends located under bridge expansion joints sometimes show distress. These joints consist of flexible seals that are needed for thermal expansion but are also prone to tears from roadway objects and material buildup. Water and salt leaking through these tears can cause corrosion and deterioration at the girder ends.
Funded by the Minnesota Department of Transportation (MnDOT), this project involved bringing bridge girders from the Trunk Highway 169 Nine Mile Creek Bridge near Edina and Minnetonka into the U’s Theodore V. Galambos Structural Engineering Laboratory for testing. Some of the girders had been repaired in 2013 using a novel method that involved removing deteriorated concrete, placing steel reinforcement cages around the damaged girder ends, and encasing the girder ends with concrete.
“Repairs were done in the field by an experienced contractor, in a colder environment and season, in a swamp,” says Paul Pilarski, metro north regional bridge construction engineer with MnDOT’s Bridge Office. While some of these field conditions could have adversely affected the results of the repair, the repaired girder ends appeared to be in good condition when the bridge was scheduled for replacement in 2017.
To verify the strength of the repairs, MnDOT selected two pairs of girders, each containing a girder that had been repaired and one that was in good condition. All four girders were brought to the U of M, where a team led by civil, environmental, and geo- engineering professor Carol Shield began its evaluation.
The research team’s goal was to determine whether the repairs had restored the damaged girders to their original shear strength. If so, MnDOT would be able to use this method—in which damaged girder ends can be repaired for $5,000 to $10,000 without causing traffic interruptions—on other bridges. The alternative involves constructing a new girder, closing traffic, removing the bridge deck over the damaged girder as well as the girder itself, and recasting the bridge deck and barrier. This replacement method costs hundreds of thousands of dollars and can require more than a month of bridge lane closures.
To test the girders, researchers began by casting a new, high-strength concrete deck for each of the four girders to recreate their field configuration. Each one was then loaded by a hydraulic ram pushing down on the girder. The researchers applied a load in 25,000-pound increments to almost 500,000 pounds on each girder. They traced cracks with colored markers on the girder ends, recording details about condition at each step of the way.
Findings indicated that the repaired girder ends were at least as strong as the unrepaired girders.
“This innovative method works remarkably well,” Shield says. “The amount of damage the crew repaired was pretty extensive. In the end, the strength of the repaired damaged girders was slightly more than the strength of the undamaged girders.”
“The fact that we tested good girders alongside repaired girders gives us a high level of confidence in this method,” Pilarski says.
Based on these study findings, MnDOT plans to continue using this repair method in similar conditions to the Nine Mile Creek Bridge. The agency will also continue to refine the repair method based on best industry practices.